This invention relates generally to volume rendering, and more particularly, to histogramming in a rendering pipeline.
Volume rendering is often used in computer graphics applications where three-dimensional data need to be visualized. For example, the volume data are scans of physical objects, or atmospheric, geophysical or other scientific models. With volume rendering, the internal structure, as well as the external surface of physical objects and models are visualized. Voxels are usually the fundamental data items used in volume rendering. A voxel is a data item that represents attributes of a particular three-dimensional portion of the object or model.
A voxel represents some particular intensity value of the object or model, such as physical parameters, e.g., density, elasticity, velocity, to name but a few, inferred by CT, MRI, PET, SPECT, ultrasound, or other scanning means. During rendering, the voxel values are converted to pixel color and opacity (RGBxcex1) values which can be projected onto a two-dimensional image plane for viewing.
One frequently used technique during the rendering process is ray casting. A set of imaginary rays are xe2x80x9ctracedxe2x80x9d through the array of voxels. For a particular viewing orientation, the rays are cast to the image plane in either a back-to-front, or a front-to-back order. The voxel values are sampled along the rays, and various techniques are known to reduce the sampled value to pixel values.
Volume rendering can be done by software or hardware. In one prior art hardware implementation, as shown in simplified form in FIG. 1, the hardware is arranged as a multi-stage pipeline 100, see U.S. patent application Ser. No. 09/190,643 xe2x80x9cFast Storage and Retrieval of Intermediate Values in a Real-Time Volume Rendering System,xe2x80x9d filed by Kappler et al. on Nov. 12, 1998. The input to the pipeline 100 is voxel stored in a voxel memory 101, and the output is pixels stored in a pixel memory or frame buffer 109. The stages can include decoding 110, interpolating 120, gradient estimating 130, classifying and shading 140, and compositing 150.
During operation of the pipeline 100, the decoder (address generator) 110 generates addresses of voxels stored in the voxel memory 101. The addresses are generated in a suitable order. Blocks of voxels are read from the memory, and presented to the pipeline for processing one at the time.
The interpolator 120 assigns values to sample points along the rays based upon voxel values in the neighborhood of the sample points. Typically, one can interpolate either voxel fields, or color-opacity fields using a predetermined interpolation mode, e.g., linearly, probabilistically, or nearest neighbor.
During gradient estimation 130, vectors (GUVW) representing the direction and rate of change of voxel or sample values are estimated. Gradients with large magnitudes denote surfaces, or boundaries between, for example, types of material. The gradients are applied to shading and illumination functions to produce highlights that enhance the three-dimensional appearance.
During classification 140, color and opacity values are assigned to each sample point. During illumination 145, also known as shading, sample points are illuminated with highlights and shadows to produce a more realistic three-dimensional appearance. For example, Phong shading can be applied, see Phong, xe2x80x9cIllumination for computer generated pictures,xe2x80x9d Communications of the ACM 18(6), pp. 49-59.
The output of the shading stage 145 is a stream of color and opacity values at sample points. This stream is fed to the compositing unit for accumulation into the pixel values of the rays. The pixel value of each ray must be accumulated one sample point at a time. Finally, after the color and opacity values of all of the sample points on an individual ray have been accumulated, the resulting pixel value of that ray is written to the pixel memory.
It will be appreciated that the order of gradient estimation, interpolation, and classification can be permuted. In fact, different prior art systems use different orders for the stages. However, in the prior art of ray-casting, the order of the stages in a particular system is fixed.
It is desired that the classification of voxels be generalized. In the prior art, voxel typically are single values or a fixed format dedicated to a specific application. Generalizing the format, will allow the pipeline to support a greater variety of applications.
A method generates a histogram of a volume data set in a rendering pipeline. The volume data set includes a plurality of samples stored in a memory. Each sample is partitioned in parallel into a plurality of field according to a plurality of format registers, there is one format register for each field.
Entries of a plurality of tables are indexed in parallel with the plurality of fields, there is one entry indexed in each table by each field. The indexed entries are incremented to accumulate a histogram of the fields of the samples of the volume data set.
The histograms can be shadow, depth, stress, distance, or gradient component maps.